The X-ray view of Giga-Hertz peaked spectrum radio galaxies

Context. This paper presents the X-ray properties of a fluxand volume-limited complete sample of 16 Giga-Hertz Peaked Spectrum (GPS) galaxies. Aims. This study addresses three basic questions in our understanding of the nature and evolution of GPS sources: a) What is the physical origin of the X-ray emission in GPS galaxies? b) What physical system is associated with the X-ray obscuration? c) What is the “endpoint” of the evolution of compact radio sources? Methods. We discuss the results of the X-ray spectral analysis, and compare the X-ray properties of the sample sources with radio observables. Results. We obtain a 100% (94%) detection fraction in the 0.5–2 keV (0.5–10 keV) energy band. GPS galaxy X-ray spectra are typically highly obscured (〈N H 〉 = 3× 10 22 cm; σNH ≃ 0.5 dex). The X-ray column density is higher than the HI column density measured in the radio by a factor of 10 to 100. GPS galaxies lie well on the extrapolation to high radio powers of the correlation between radio and X-ray luminosity known in low-luminosity FR I radio galaxies. On the other hand, GPS galaxies exhibit a comparable X-ray luminosity to FR II radio galaxies, notwithstanding their much higher radio luminosity. Conclusions. The X-ray to radio luminosity ratio distribution in our sample is consistent with the bulk of the highenergy emission being produced by the accretion disk, as well as with dynamical models of GPS evolution where X-rays are produced by Compton upscattering of ambient photons. Further support for the former scenario comes from the location of GPS galaxies in the X-ray to O[iii] luminosity ratio versus NH plane. We propose that GPS galaxies are young radio sources, which would reach their full maturity as classical FR II radio galaxies. However, column densities ∼>10 22 cm could lead to a significant underestimate of dynamical age determinations based on the hotspot recession velocity measurements.